Over the last few decades, the semiconductor industry has undergone a revolution by the use of semiconductor technology to fabricate small, highly-integrated electronic devices, and the most common semiconductor technology presently used is silicon-based. A large variety of semiconductor devices have been manufactured having various applications in numerous disciplines. One silicon-based semiconductor device is a medal-oxide-semiconductor (MOS) transistor. The MOS transistor is one of the basic building blocks of most modern electronic circuits. Importantly, these electronic circuits realize improved performance and lower costs, as the performance of the MOS transistors increased in as manufacturing costs are reduced.
A typical MOS device includes a bulk semiconductor substrate in which a gate electrode is disposed. The gate electrode, which acts as a conductor, receives an input signal to control operation of the device. Source and drain regions are typically formed in regions of the substrate adjacent the gate electrodes by doping the regions with a dopant of a desired conductivity. The conductivity of the doped region depends on the type of impurity used to dope the region. A typical MOS device is symmetrical, and the source and drain are interchangeable. Whether a region acts as the source or drain typically depends on the respective applied voltages and the type of device being made. The collective term source/drain region is used herein to generally describe an active region used for the formation of either a source or drain.
As an alternative to forming a MOS device on a bulk semiconductor substrate, a semiconductor layer can also be formed on an insulating substrate, or over an insulation layer formed in a semiconductor substrate. The insulating layer is often referred to as a “buried oxide layer”, since oxide is typically employed as the insulation layer. This technology is generally referred to as silicon-on-insulator (SOI) technology. If silicon germanium is employed as the semiconductor layer formed on the insulation layer, the technology is referred to as SOI technology. In the following, both of these different technologies will be referred to as simply SOI technology for ease of discussion.
SOI technology offers potential advantages over bulk materials for the fabrication of high performance integrated circuits. For example, dielectric isolation and reduction of parasitic capacitance improve circuit performance. Also, compared to bulk circuits, SOI is more resistant to radiation.
It is known that diode ideality is impacted by defects at the buried oxide/silicon interface. Consequently, the thinner the silicon layer, the less ideal is the diode. In thin silicon layers, transport mechanisms other than drift diffusion can dominate the diode, causing a defect-generated leakage circuit.
Although a thicker silicon layer allows for improved diode ideality, it is still important to provide thin regions of silicon for active regions of devices, such as transistors.